1. Field of the Invention
The present invention relates to an optical transmitter, in particular, the invention relates to a transmitter used in an optical communication system with a long reach.
2. Related Prior Art
The long distance communication is currently limited to 80 to 100 km for the transmission speed of 10 Gbps without any repeater and dispersion compensation. A report of IEEE Photonics Technology Letters, 2006, vol. 18(2) by Mahgerefteh et al., has proposed one of techniques to extend the transmission range, which is called as the chirp managed laser (hereafter denoted as CML) technique.
An optical transmitter implementing the CML technique suppresses the transition chirp by applying relatively larger bias current and smaller modulation current to a laser diode (hereafter denoted as LD) and obtains a substantial extinction ratio of the output from the transmitter by filtering a component corresponding to the signal “0” with an optical filter having a sharp cut-off characteristic.
Such a filter with the sharp cut-off may be a Fabry-Perot Etalon filter with a periodic transmission spectrum, that is, the Fabry-Perot Etalon filter has a plurality of transmission maxima. Accordingly, it may occur that the emission wavelength of the LD is not always matches with one of the cut-off frequencies of the filter. Moreover, when the emission wavelength matches with one of cut-off frequencies of the filter to cut shorter frequencies, the filter cuts the component “1”, which not only degrades the extinction ration of the transmitter but reverses the phase of the optical signal.